Motor control may have a wide range of applications. The applications may include, among others, electrical motors for electric vehicles, hybrid cars but also electrical motors for residential washing machines, fans, hand-held power tools, industrial motor drives, etc.
Induction or asynchronous motors are typically used in the above mentioned applications. Induction or asynchronous motors are AC motors in which a current is induced by electromagnetic induction in a rotating winding by a magnetic field generated in a static winding. Induction or asynchronous motors do not require sliding electric contacts electrically connecting the rotating winding to the static winding, thereby simplifying construction and improving reliability of the induction or asynchronous motors.
Availability of affordable, reliable power transistors (e.g., power MOSFETs and IGBTs) and modules capable to drive such induction or asynchronous motors are an important design goal in the above mentioned applications.
In three phases induction or asynchronous motors, so-called half bridges or full bridge topologies are commonly used. In half-bridge or full bridge topologies six power transistors may be used, three power transistors for a top side of the bridge and three power transistors for the bottom side of the bridge. Each power transistor of the top or bottom side of the bridge is arranged with another power transistor of the bottom or top side of the bridge in an inverter pair topology. Each pair of power transistors is in this away arranged to be used for each phase of the induction or asynchronous motor.
Typically, a gate drive circuit controls a gate of the power transistor in a way that when one power transistor of the inverter pair is on the other power transistor of the inverter pair is off.
Each inverter pair is typically supplied by a high voltage supply which may be in the example of electric vehicles applications the high voltage supply generated by a battery pack (e.g. a Lithium Ion battery pack) used to drive the electric motor of the electric car. This high voltage supply may be in the range of 200 to 300 Volts. The gate drive circuit is typically powered by a lower supply voltage derived and isolated from the high supply voltage. For example, it is common to use transformers (e.g. DC-DC converters) to generate the lower supply voltage used to supply the gate drive circuit.
There are however a few problems associated with the use of transformers in the above mentioned applications. One problem is that transformers are big and expensive components that take up a significant amount of the module space in which the gate drive circuits and the power transistors are also implemented. Another problem associated with the use of transformers is that transformers have typically parasitic capacitors between a primary winding and a secondary winding. By way of an example if the primary winding is connected to the high voltage supply and the secondary winding is connected at the low voltage supplied circuits, a fast and large change of an high output voltage during commutation may inject parasitic currents to the secondary winding through the parasitic capacitors. This in turn may negatively affect the gate drive circuit and all other circuitry connected to the low voltage supply at the second winding.